1. Field of the Invention
The present invention relates to a hybrid construction machine which uses both engine power and electric power stored by an electrical storage device.
2. Description of the Related Art
A background art of the present invention will be described based on an example of an excavator with reference to FIG. 5.
The excavator includes a crawler-type lower traveling body 1, an upper slewing body 2 mounted on the lower traveling body 1 slewably around an axis X perpendicular to a ground surface, a front attachment 3 attached to the upper slewing body 2 for an excavation operation, and an engine. The front attachment 3 has a boom 4 capable of being raised and lowered, an arm 5 connected to a front end of the boom 4, a bucket 6 connected to a front end of the arm 5, and a plurality of hydraulic actuators for driving the boom 4, the arm 5, and the bucket 6, namely, a boom cylinder 7, an arm cylinder 8 and a bucket cylinder 9.
In addition, described in Japanese Patent Application Laid-open No. 2000-283107 and Japanese Patent Application Laid-open No. 2007-247230 are hybrid excavators, each of which further includes: a hydraulic pump for driving the hydraulic actuators, a generator-motor which performs an electric generator action and an electric motor action, and an electrical storage device. The hydraulic pump and the generator-motor are connected to the engine. The generator-motor charges the electrical storage device by the electric generator action, and on the other hand, timely driven by use of electric power stored in the electrical storage device to perform the electric motor action, thereby assisting the engine.
Furthermore, there is a hybrid excavator further including an electric actuator, which is actuated by use of the electrical storage device as an electric power source, such as a slewing electric motor as a slewing driving source of the upper slewing body 2, an electric cylinder which actuates the front attachment 3, or the like. Such an electric actuator is able to make a regeneration operation. The electric actuator, if being, for example, a slewing electric motor, generates regenerative electric power involving a slewing deceleration operation. The generated regenerative electric power is stored in the electrical storage device. This charge action of the electrical storage device causes a regeneration brake.
This type of hybrid excavator, however, has a possibility of overcharge of the electrical storage device in excess of a proper charge rate due to recovery of the regenerative electric power, in the case where the electrical storage device has a high charge rate when recovering the electric power, and the overcharge can involve a failure and life shortening of the electrical storage device. The overcharge may be prevented by providing a regenerative resistor to consume the regenerative electric power of the electric actuators; however, this requires addition of further facilities separately from the regenerative register, namely, a relatively large regenerative resistor, a control device for the regenerative resistor, and a further facility for treating heat generated by the regenerative resistor. This makes devices be difficultly arranged in the excavator with a severe space limit and involves substantially increased cost.